Method of making piezoelectric units



Patented Mar. 29,

2,112,636 p ME'rnon or MAKING rmzoELEoTnio Um'rs .Charlesv Sawyer, Charles H. Tower, and Alfred L. W. Williams, Cleveland, Ohio, assignors to The Brush Development Company,

Cleveland,

Ohio, a corporation of Ohio v Application September 2, 1936, Serial No. 99,154

. l 10 Claims. This invention relates to piezo-electric crystal units and more particularly to methods of pro-` ducing such umts.

'I'he object of the invention is to 4provide a simple and emcient method of producing a piezoelectric unit having one or more relatively thin plates or elements of Rochelle salt or similar material.

Piezo-electric units of a type employing a plate of Rochelle salt or similar material cemented to a restraining body to oppose the piezo-electric deformations of the plate are now well known. U. S. Patent 1,803,274 discloses a unit of the'exing type employing a plate of crystalline material and a non-piezo-electric plate cemented together, the non-piezo-electric plate acting as a restraining body to oppose the piezo-electric deformations ofthe crystalline plate. U. S. Patents Reissue 20,213 and 1,803,275 disclose flexing units employing two or more crystalline plates cemented together in opposition, each plate acting as a restraining body for the other plate. Sawyer and Tower in Physical Review, Vol. 35, No. 3, February 1, 1930, pages 269 to 273, have shown that this restrained crystal construction reduces temperature and saturation effects.

In many applications of piezo-electric material, especially of Rochelle salt plates, it is essential that the plate be made very thin. Indeed for some applications the most desirable results are secured with plates having a thickness of from .003 to .004 inch. However, prior to' the present invention the difficulties of constructing such thin plates and units embodying such plates were exceedingly great because of the dimculties of reducing the plates to the desired degree of thinness and the great tendency of the thin plates to fracture in applying electrodes to them-and in cementing them to each other or to other bodiesv especially where considerable pressure was necessary to drive out excess cement between the plates or to force the plates into close contact.

In order to overcome these difficulties we have devised the following method. Relatively thick plates of Rochelle salt with parallel faces are first formed from slabs of such crystalline material.

This can be done by sawing the slabs into the` plate form and milling the sawed plates to uniform thickness such as to afford suillcient rigidity Vand strength to withstand the stresses incident to the handling, electroding and cementing of the plates. A thickness of about 1x6 of aninch has been -found satisfactory in most cases, the optimum thickness depending somewhat on the vother dimensions of the plate. Assuming that a unit is to be constructed comprising a single plate of crystalline material and a non-piezo-electric restraining body as, for example, in U. S.,Patent 1,803,274, an electrode is applied to' one face of. a relatively thick piezo-electric plate. A layer of cement is then applied to this electrode surface or to the restraining body or to both and the two surfaces are brought into contact and suicient pressure is applied to reduce the layer of cement to the desired thinness and obtain the required adhesion between the plates. Due to the thickness of the crystalline plate it will withstand the forces required in this operation. The

cement is then allowed-to set. In case units are to be constructed employing two or more crystalline plates as shown in the aforesaid Patents Reissue 20,213 and 1,803,275, one or both of the electroded surf-aces of two crystalline plates are coated with cement and brought together with the application of pressure to squeeze out the excess cement, and in case the unit is to be made without inner electrodes the proper surfaces are simply coated with cement and brought together under pressure and the cement allowed to; set.

,To reduce the crystalline plate or plates of the unit to the proper thinness we provide suitable apparatus comprising a support having a at attached and a cutting tool adapted to engage and remove crystalline material from the exposed face of the unit. We prefer to remove the surplus crystalline material by a turning operation and to this end we provide a rotatable disk having a at face which is very true in a plane perpendicular to the axis of rotation and having a rim projecting from the surface of the disk a few thousandths of an inch. This rim preferably is a separate part movably mounted on the disk. The crystal unit is attached toV the fiat 'face of the disk preferably by placing one face of the unit against the face of the disk and causing the exposed face of the unit to be subjected to a substantially greater atmospheric pressure than -surface to which one face of the crystal unit is the other faceI thereof so that atmospheric pressure it utilized in holding the crystal unit on the disk. 'I'his can be accomplished by providing air recesses and passages in the disk structure by means of which air may be drawn away from pacity of the surface of the disk may be set on.

in this manner. The disk is then revolved at' suitable speed and a citting tool with the surfaces whose intersection forms the cutting edge making quite a steep angle with the plane of the face of the disk is preferably brought in contact with the edges of the crystalunits nearest the center and gradually fed towards the periphery of the disk and parallel .to the surface thereof. The tool may if desired be fed from the periphery toward the center but a sufliciently light cut should be taken to prevent `forcing the units in toward the center. In practice an average cutl ting speed of about 2000 feet per minute has been found satisfactory. The angle leading from the cutting edge of the tool may be very much more acute than is the common practice in turning operations because not much eort is required to remove the-somewhat friable crystalline material so that there is only a moderate amount of heat to be carried away. When the edge of the tool has reached the edge of the disk a known amount of crystalline material will have been removed from the surfaces of the crystal units. This cutting process may be repeated until sumcient crystalline material has been removed from the outermost surfaces of the units to bring the plates to the desired degree of thinness.

If the crystal -units are of the multiple plate type they are then removed from the disk and replaced thereon with the faces which have :lust` been operated upon next tothe face of the disk. The turning process is now repeated until the exposed surface is brought tothe same distance from the cemented junctions as are the opposite surfaces of the units. The thin cemented crystal units are now removed from the surface plate. These units with their laminated structure, .though thin, are strong enough to withstand the handling incident to the removal of the grease and the application of electrodes to their outer surfaces. l

It is to be observed that the viscous coating employed as above described serves not merely to prevent access of air to the supported side of the crystal plate or unit during the cutting operation thus utilizing the atmospheric pressure on the other face to hold the unit and disk in contact, but also, by filling in any slight d ei pressions or irregularities in the surface of the crystal plate, to form a continuous support for the surface of said plate so as tol minimize breakage of the crystalline plates during the machiningoperation. In addition, the viscous material of course also helps by adhesion to hold .the crystal plate in position on the rotating disk.

As further illustrating and explaining the invention we have shown .in the accompanying drawing several specific embodiments of our improved piezo-electric crystal units and turning apparatus for use in carrying out our improved method of producing the units.

In the drawing.

Figs. land-2 are face and edge views, respectively, of a relatively thick plate of Rochelle salt crystal such as is formed in the manner above described, a portion of the plate being shown in section in Fig. 2.

Fig. 3 is an edge'view of a relatively thin v method, a portion of the plate being shown in' section.

Fig. 4 is an edge view, partly in section, of an unfinished crystal unit consisting of two such plates as are shown in Figs. 1 and 2, with electrodes applied to their adjacent faces and with the two plates cemented together, the space between the two plates .being exaggerated in order to more' clearly show the electrodes.

Fig. 5 is an edge view, partly in section, of the same" crystal unit after one of the plates has been turned down to the desired degree of thinness.

Fig. 6 'is a 'similar edge view of the same unit after both plates have been turned down to the desired degree of thinness.

' Fig. 'I is an edge view, partly in section, of the same crystal unit after electrodes have been applied to the outer faces of the crystal plates.

Figs. 8 and 9 are, respectively, face and edge views of a plate of Rochelle salt crystal similar to that shown in Figs. land 2,-the plate being shown partly in section in Fig. 9.

Fig. 10 is an edge view, partly in section, of a partially completed piezo-electric unit consisting of a crystal plate such as is shown in Figs. 8 and 9 cemented to the face of a metal plate.

Fig. 11 is an edge view, partly in section, of the same unit with the crystalplate turned downto the desired degree of thinness.

Fig. i2 is an edge view, partly in section, of the same unit with an electrode applied to the exposed face of the Icrystal plate and electric leads attached to the electrode and the metal plate, respectively.

Figs. 13 and 14, are, respectively, a front or 'side elevation, partly in section, and a face elevation of a rotatably mounted disk of the character previously referred to adapted for use in turning down the crystal plates to the desired degree of thinness. a number of crystal units being shown on the disk to better illustrate the turning operation.

Referring in detail to the structures illustrated in the drawing. and rst to the crystal illustrated -in Figs. 1 to 3, I is a relatively thick rectangular plate cut from aRochelle salt crystal or slab 'as previously described and I a represents the plate after it has been turned down to the desired degree oifv thinness by our improved method.

Referring to the production of the piezo-electric unit shown in Fig. 7,v as that production is illustrated by Figs. 4 to 7, inclusive, each of two Rochelle salt crystal plates 2, 2 of thesame character as the plate l, has an electrode l applied to one of its faces. The two electroded faces of the two plates are then cemented together with an electrode lead 3a between them as indicated inFig. 4. As the plates 2,2 are relatively thick and correspondingly strong, the application of the electrodes to their surfaces and the cementing together of the two plates can readily be effected without serious danger of breakage of the plates. One of the two plates is then turned down to a desired degree of thinness, as represented by plate 2a in Fig. 5. Then the other plate 1 the two leads being brought together as indicated in Fig. 7. During the reduction of thickness of the twocrystal plates each is supported and strengthened by the other. It will be understood that the thickness of the plates of the iinished unit may be made much smaller than shown in the drawing, -a total thickness for the unit of .020 inch or less being easily obtained.

In the production lof the piezo-electric unit shown in Fig. l2 some modication of the above described procedure is involved. Here a relatively thick rectangular plate 5 of Rochelle salt crystal, of the same character as the plates I and 2 above described, has one face cemented to'the face of a restraining member in the form of a rectangular metal plate of the same size, producing an unfinished unit as shown in Fig. l0. A portion of the crystalline material of the plate 5 is then removed to reduce the thickness of said plate, leaving the unit in the form shown in Fig. 11 in which the crystal plate is represented by the numeral 5a. Finally, an electrode 'l is applied to the outer face of plate 5a and a lead la attached to the unit. The metal plate 5 itself serves as an electrode on the opposite side of plate 5a and has a lead 6a attached thereto. Alternatively, an electrode may be applied to the inner face of the thick plate .5 before the latter is cemented to the metal plate 6 and this is preferable in at least some instances as giving closer contact between the electrode and the crystal plate.

Referring now to the turning apparatus shown in Figs. 13 and 14, B is a disk having integral or rigid therewith a shaft 9 which can be rotatably mounted in any suitable bearing supports (not shown) and driven in any desired manner by power. The disk il is formed with a smooth face 8a constituting substantially a true plane surface at right angles to the axis of rotation of the disk. A rim IU is mounted on the periphery of the disk 8 with a loose t. In the normal position of the rim II) one edge oi' it projects beyond the surface or face 8a of the disk. The rim I is held in such normal position by means of screws il, Il which extend through slots Illa in the rim. The screws are provided with knurled heads and by loosening them the rim I0 can be moved on the disk B so as not to extend beyond the face 8a.

At i2 is shown a turning tool adapted to cooperate with the rotatably mounted disk 8. The tool I2 can be mounted in any well known manner upon a tool support such as is employed in turning lathes and which is adapted to be adjusted manually and fed mechanically parallel to the face of the disk 8.

In the use of the turning apparatus, the face 8a of the disk iscovered with a coating of some suitable viscus substance, such as has been referred to. Then the disk is loaded withthe work to be turned down by selecting units such as those shown in Figs. 2, 4 or 10, and pressing such units firmly into the viscous coating of the face 8a of the rotatable disk. As shown in Fig. 13, the disk hasbeen loaded with seven units I3, i3 of the character illustrated in Fig. 4. The tool I2 having been set near the center of the disk and manually adjusted the right distance from the face 8a of the disk, the machine is started with the disk rotating in clock wise direction as indicated by the arrow in Fig. 14 and with the feed movement of the tool toward the periphery of the disk and parallel to the face thereof. As the tool engages the crystal units the crystalline material is gradually cut away as will readily be understood. It is f found that the crystal units adhere strongly to vbreakage of the crystalline units.

the face of the rotating disk, any movement of the work in relation to the disk being parallel to the face thereof. Under the light pressure of the tool and the centrifugal force of the rotating units there tends to be a creeping of the crystal units toward the periphery of "the disk but any moveproper is prevented by the rim I0.

When the cutting tool has progressed to the rim ofthe rotating disk, if more material is to be removed from the same side of the crystal units the tool is reset near the center of the disk and the cutting operation repeated. In this manner the desired amount of crystalline material can be removed. When the desired amount of material has been removed from one side of the unit, the securing screws II, II are loosened and the rim I2 slipped back on the disk 8 so that the crystal units can be slid radially outward oi thev face of the disk. The adherence of the crystal units to the face of the disk is so strong that it would be diloult otherwise to remove the units.

To complete these same crystal units they'are removed from the disk in the manner stated and replaced with their thin crystal plates against the disk so that their thicker plates are in Aposition to be engaged by the cutting tool. 'I'he thicker lplates are then turned down, in the manner described above, to the same thinness as the other plates.

We have deemed it unnecessary to illustrate crystal units having more than two crystal plates and to describe the application o1' our process to such units and plates since it is clear and obvious that the process can advantageously be used to produce such units, the essential principle involved in all cases in which a unit is to be made comprising a thin crystalline plate cemented 4to another member or body, being that a relatively thick plate of the crystalline material is cemented to the other member (either another thick plate of the crystalline material or a non-piezo-electric restraining member) and is then reduced to the desired thin form while the unit is supported by the other member. Thus, if four-plate units such as are described in United States Letters Patent ment of the work beyond the periphery of the disk 1,803,275 are to be produced, one possible procedure is to produce a two-plate unit in the manner already described and then to cement crystal plates to the faces of the two-plate unit and turn them down to the desired thinness by the same sort of procedure. Such a method of producing the four-plate unit obviously embodies the above noted principle of our method. It will also be obvious that still other ways of proceeding, embodying our basic principle, can be followed to produce units having more than two elements.

It should be understood that in its broader aspects our improved method is not limited to the employment of a turning operation to reduce the thickness of the crystalline plates. Obviously the crystalline material can be removed by a milling operation, a shaping operation or other suitable machining operation and in any such case the support to the face of which the crystal unit .is attached may be stationary and the cuttersthe preferable procedure because 'it is simple, 7

rapid and effective and minimizes cracking and Since, inthe turning operation, the tool has contact with a given point on the crystalline surface for a comparatively short length of time and this part of the-crystalline surface has a chance to dissipate ing and breakage of the units from unequal vexpansion of the crystalline material are largely obviated. It should be further understood that while we have described our method as applied to the production of Rochelle salt units, it is equally adapted to production of units of other mechanically similar pieno-electric material. y

By the process which we have dcribed we are able to produce on a commercial basis and with but slight loss from breakage large quantities of crystal' units of Rochelle salt or the like having crystalline plates-as thin as two and one half to three thousandths of an inch. and are able to produce very thin single plates of crystalline materi'al and maintain thethickness of these units and plates within very strict limits so that they may be used interchangeably in various pieces of apparatus. By means of our improved process it is possible to make the individual crystal plates of multiple-plate units such as are shown in Pigs'. 'i and l2 thinner without undue breakage than is possible where individual plates alone areturned downtothinform. This, ashas been explained, is due to the fact that in our improved method of producing the multiple-plate units each plate supports and strengthens the other during the operations of cutting and application of external electrodes.

Obviously the pieno-electric units produced by our method may take various forms in addition to those illustrated. Por example, a unit similar to that shown in Pig. "l may be produced without the lead la and with entirely separate leads attached to the external electrodes l, l this being a known form of units oi' this character. Similarly such two-plate units may be made with the complete omission of the internal electrodes l, 3 entirely separate leads in this case also being provided i'or the outer electrodes l, I. Likewise the -method of producing the units can be modified in various ways, the scope of 'the' invention being indicated by the appended claims.

What we claim is:

l. The method of making a piezo-electric unit comprising one or more thin plates of frangible t crystalline material which comprises forming a relatively thick plate of said material, permanently cementing one face of the plate to the surface of another body, and effecting relative movement between the resulting unit and a cutting tool to cut away crystalline material from the l exposed face of the crystal plate until the latter is reduced to the desired'degree of thinness.

2. A method of forming a built-up piezo-electric unit comprising two or more thin plates of frangible crystalline material which comprises cementing together face to face two relatively thick plates of the said material, supporting the resulting unit by the first of said thick plates,

\ while said units is so supported removing crystalline material from the exposed face of the second thick plate until the latter is reduced to a material and a -body of non-pieao-electric material which comprises forming a relatively thick plate of the crystalline material, permanently cementing one face `of the said thick plate to the surface ofv the non-pieao-electric body, and effecting relative movement between theresulting unit and a cutting tool to cut away crystalline material from the exposed face of -the crystal plate until the latter is reduced to the desired degree of thinness.

4. The method of making a piezo-electric unit comprising one or more thin plates of frangible crystalline material which consists in forming a relatively thick plate of said material, permanently cementing one face of the plate to the surface of another body with electrode material at the Junction of the said face and surface, eilecting relative movement between the resulting unit and a cutting tool to cut away crystalline material from the exposed face of the crystal plate until the latter is reduced to the desired degree of thinness, and thereafter applying an electrode to the exposed fac'e of the thin crystalline plate.

45l The method of making a piezo-electric unit comprising a plurality of thin plates of frangible crystalline material which comprises'cementing togetherA face to face two relatively thick plates of the said material, supporting the resulting unit by one of the plates while removing crystalline material from vthe exposed face of the other plate until the latter is reduced to the desired degree-of thinness, and, while supporting the said unit by the thinner plate, removing crystalline material from the exposed face of the thicker plate until the latter also is reduced to the desired degree of thinness.

6. Ihe method of making a piezo-electric unit comprising a plurality of thin plates of frangible crystalline material which comprises cementing together face to face two relatively thick plates of the said material, supporting the resulting" unit by one of the plates while removing crystalline material from the exposedface of the other plate until the latter is reduced to the desired degree oi' thinness, and, while supporting the said unit by the thinner plate, removing crystalline material from the exposed face of the thicker` plate until the latter also is reduced to the desired degree of thinness, and applying electrodes to the exposed faces of the two-plate unit.

7. The method oi' making a piezo-electric unit comprising a plurality of thin plates of frangible crystalline material which comprises cementing together face to face two relatively thick plates of the said material, positioning one face of the resulting crystalline unit on a support with the exposed face of the unit subjected to a. greater atmospheric pressure than the other face thereof to hold the unit on the support, effecting relative motion between the support and a. cutting tool to cut away the crystalline material of the exposed face of one of the crystalline plates until it is reduced to the desired degree of thinness, thereafter positioning the crystal unit on the support with a face of the unreduced thick plate exposed, and,'while holding it thereon by atmospheric pressure on its exposed surface, cuttingv away crystalline material of the exposed face of the thicker crystalline plate until the latter also is reduced to the desired' degree of thinness.

8. The method of making a piezo-electric unit comprising a plurality of thin plates of frangible crystalline material which comprises cementing together face to face two relatively thick plates crystal unit on the support with a face of the unreduced thick plate exposed, and, while holding of the said material, pressing one face of the resulting crystalline unit against the plane face fof a support after having rst applied tov one of said faces a coa-t of viscous substance, eecting a relative motion between the support and a cutting tool to cut away the crystalline material from the face of the exposed crystalline plate until it is reduced to a desired degree of thinness, thereafter positioning the unit upon the support with a face of the unreduced thick' plate exposed and with a coating of viscous substance between the unit and support, and again eie'cting relative motion between the support and vthe cutting tool to cut away crystalline material o! the exposed face of the thicker plate of theunit until said plate also is reduced to a desired degree of thinness.

9. The method of making a piezo-electric unit comprising a plurality of thin plates of franaible crystalline material which comprises applying an electrode to one face of each of two relatively thick plates of the said material, cementing together the -two electroded faces of said plates, positioning one face of the resulting crystalline unit on a support with the exposed face of the unit subjected to a greater atmospheric pressure than the other face thereof to hold the unit on the support, eiIecting relative motion between the support and a cutting tool to cut away the crystalline material' of the exposed i'ace of one of the crystalline plates until it is reduced to the desired degree of thinness, thereafter positioning the ol' thinness.

it thereon by atmospheric pressure on its exposed surface, cutting away crystalline material of the exposed face of the thicker crystalline plate until the latter also is reduced to the desired degree of thinness.

10. The method of making a piezo-electric unit comprising a plurality of thin plates of frangible crystalline material which comprises applying an electrode to one face of each of two relatively thick plates of the said material, cementing together the two electroded faces of said plates, pressing one face of the resulting crystalline unit against the plane face of a support after having rst applied to one cf said faces a coat of viscous substance, eiecting a relative motion between the support and a cutting tool to cut away the chystalline material from the face of the exposed crystalline plate until it ls reduced to a desired degree o thinness, positioning the unit upon the support with a face of the unreduced thick plate exposed and with a 'coating of viscous subst-ance between the unit and support, and again eecting relative motion between the support and the cutting tool to cut away crystalline material of the exposed face of the thicker plate of the unit until said plate also is reduced to a desired degree CHARLES B. SAWYER. CHARLES H. TOWER. ALFRED L. W. WILLIAMS. 

